Conventionally, in forming images using toner, electrophotography has been generally used, i.e., the application of the Carlson process. The principle of electrophotography is described in detail in reference to FIG. 7 through an example of the normal developing system adopted in photocopying machines. In the photocopying machine which employs the Carlson process, a charger 32, an exposure unit 33, a developer unit 34, a transfer unit 35, a fuser 36, a cleaner 37, and an eraser 38 are provided in this order along the circumference of a photoreceptor drum 31 having a photosensitive layer formed on the surface thereof as shown in FIG. 7.
With this arrangement, first, the surface of the photoreceptor drum 31 is uniformly charged by a charger 32 in a dark place. Next, an original image is illuminated on the surface of the photoreceptor drum 31 by the exposure unit 33 so as to remove charges from the illuminated portion, thereby forming an electrostatic latent image on the surface of the photoreceptor drum 31. Thereafter, toner 39 is made to adhere to the electrostatic latent image, the toner 39 being charged by applying thereon a charge with a polarity opposite to the charge on the photoreceptor drum 31 in the developer unit 34, thereby forming a visible image with the toner 39.
Further, a copying material 40 is superimposed on the visible image. Then, a corona-discharging is carried out by the transfer unit 35 from the back surface of the copying material 40 so as to apply a charge with a polarity opposite to the toner 39. As a result, the visible image is transferred onto the copying material 40. Then, using heat and pressure from the fuser 36, the transferred visible image is made permanent on the copying material 40. Any residual toner 39a remaining on the photoreceptor drum 31 after the transfer are removed by a cleaner 37. After eliminating the charge from the electrostatic latent image on the photoreceptor drum 31 by projecting thereon a light beam from the eraser 38, the process starting with the charging process by the charger 32 is repeated, thereby successively forming images.
In the discussed electrophotography wherein the Carlson process is applied, normally a corona discharger is adopted for charging the photoreceptor drum 31 or transferring the toner 39 to the copying material 40. However, when the corona discharger is adopted, the application of high voltage of several kV is required. Moreover, it is likely to be affected by a change in the ambient condition, for example, a change in the charge amount on the surface of the photoreceptor drum 31 due to a temperature change. Furthermore, ozone produced in the process of corona discharging results in problems concerning environmental health.
In order to counteract the above-mentioned problems, an image forming process has been developed, one that does not require the corona charging. This process is disclosed in Japanese Examined Patent Publication 4900/1990 (Tokukouhei 2-4900). When adopting this method, as shown in FIG. 8, a photoreceptor 50 is desirably arranged such that a transparent electrically conductive layer 52 made of In.sub.2 O.sub.3, etc., a photoconductive layer 53 made of Se etc., and a dielectric layer 54 made of polyethylene terephtalate film are laminated in this order on a transparent base 51 made of glass or the like.
In this arrangement, first a magnet 56 as a toner holder with electrically conductive and magnetic toner 55 adhering thereto is brought close to the surface of the photoreceptor 50 and voltage is then applied across the magnet 56 and the transparent electrically conductive layer 52. In this state, when exposure is conducted onto the surface of the photoreceptor 50 from the side of the transparent base 51, the electric resistance of the surface of the photoconductive layer 53 is lowered at the illuminated portion; thus, a charge is injected under the dielectric layer 54. Accordingly, a strong electric field is exerted between the magnet 56 and the photoreceptor 50 in such a manner that a charge with the polarity opposite to that of the charge injected under the dielectric layer 54 is injected to the toner 55 located in the exposed portion.
As a result, by making pairs with the charges of the opposite polarities, the charged toner 55 and the charge injected under the dielectric layer 54 through the transparent electrically conductive layer 52 become attracted to one another with the dielectric layer 54 situated in between. In this way, even after the magnet 56 has been moved away from the photoreceptor 50, the toner 55 at the exposed portion remain on the surface of the photoreceptor 50.
As described, the discussed method enables a visible image to be formed on the surface of the photoreceptor 50 without using the corona charging. After the visible image is formed on the surface of the photoreceptor 50, the visible image is transferred from the surface of the photoreceptor 50 to the surface of the copying material as in the case of the Carlson process. Thereafter, the copying material is transported to the fuser, where the toner is melted and fixed thereon by heat treatment; thus, the visible image is permanently affixed to the copying material.
However, in the case as described above, where the surface of the photoreceptor is charged by the electrically conductive and magnetic toner 55, the charged state of the surface of the photoreceptor is easily affected by those factors such as: electric resistance value of the electrically conductive toner 55 or the magnet 56; charge-holding characteristic of the surface of the photoreceptor 50; distance and potential difference between the electrically conductive and magnetic toner 55 or the magnet 56 and the photoreceptor 50; and contact area and relative speed between the toner 55 and the photoreceptor 50.
For example, the following description will discuss an experiment and its results, which was conducted about the charge of the surface of a photoreceptor. A photoreceptor, which measures 30 mm in outer diameter, is provided with a transparent base and a photoconductive layer made of amorphous Si having a thickness of 3 .mu.m and a transparent electrically conductive layer, both formed on the transparent base in this order from the surface side. Here, no dielectric layer is formed on the surface of the photoreceptor. In the state where a toner holder with electrically conductive and magnetic toner (1.times.10.sup.7 .OMEGA.cm) adhering thereto is in contact with the surface of the photoreceptor, a voltage of 20 V, designated by V.sub.s, is applied across the toner holder and the transparent electrically conductive layer of the photoreceptor while the photoreceptor is being rotated at a peripheral speed of 30 mm/s. If the surface electric potential of the photoreceptor before passing by the area in contact with the electrically conductive and magnetic toner is virtually zero, the electric potential of a particular portion of the photoreceptor is increased every time it passes by the area in contact with the electrically conductive and magnetic toner, as is shown in FIG. 9. In FIG. 9, V.sub.D.spsb.1 represents the surface electric potential of the photoreceptor after passing by the area in contact with the electrically conductive and magnetic toner once; V.sub.D.spsb.2 represents that after passing by the contact area twice; and V.sub.D represents the saturation value of the surface electric potential of the photoreceptor.
In this arrangement, the surface electric potential of the photoreceptor is merely increased to approximately 60 to 80 percents of the voltage V.sub.s that has been applied to the toner holder, after passing by once the area in contact with the electrically conductive and magnetic toner. After passing by the area in contact with the electrically conductive and magnetic toner three times, the surface electric potential is increased to the vicinity of the saturation value V.sub.D.
For this reason, as shown in FIG. 10, when the surface electric potential of the photoreceptor is lowered to a residual electric potential, designated by V.sub.R, during exposure, the potential difference of a surface portion of the photoreceptor subjected to the lowering is merely increased up to V.sub.DR after the portion in question has passed by the area in contact with the electrically conductive and magnetic toner once in the following developing process. Therefore, there exists a gap between the saturation value V.sub.D of the surface electric potential of the photoreceptor and the surface electric potential V.sub.DR of the photoreceptor after passing by the area in contact with the electrically conductive toner. This results in an electric field between the toner holder and the surface of the photoreceptor; therefore, the toner adheres to the surface of the photoreceptor even at a non-image portion that has not been subjected to the exposure. Thus, a problem is presented in that an image pattern that was formed in the exposing process of the previous rotation may be developed in the following rotation, resulting in a residual image.